Although elemental semiconductors such as silicon have dominated the commercial market, compound semiconductors such as GAAs and InP have important uses in present and future electronics. The technology for Si has achieved startling results. Millions of transistors can be fabricated in a single integrated circuit at very low cost. The technology for GaAs has been developed over many years and is finding widespread application in very high-speed electronics. As a result, GaAs substrates have become relatively inexpensive and GaAs electronic circuitry has been integrated with thousands of transistors. Compound semiconductors find another important use in opto-electronics. While GaAs can implement optical devices around the 0.8 .mu.m band, silica optical fibers have created an intense demand for opto-electronic integrated circuits (OEICs) in the 1.3 to 1.5 .mu.m band. This band cannot use GaAs but requires other materials, such as the III-V semiconductor InP.
The recent activity in InP has greatly advanced its technology. However, the cost of InP substrates of the required quality significantly exceeds that of GaAs substrates. Furthermore, the complexity of fabricated InP electronic integrated circuits greatly lags behind that for GaAs ones and probably will continue to. As a result, there is a desire to combine different semiconductor materials, the principal ones being Si, InP, and GaAs and the related families of the latter two.
In heteroepitaxy, one material is epitaxially grown on a substrate of another material, e.g., InP on a GaAs substrate. In most cases, the two materials have the same zinc-blende crystal structure, but the lattice constants are substantially mismatched. Exceptional cases of lattice matching include the ternary InGaAs alloy and the quaternary InGaAsP alloys that are lattice matched to InP and the nearly lattice-matched AlAs and GaAs. However, the lattices of InP and GaAs are mismatched by 3.7%. The typical lattice mismatch results in a density of threading dislocations that cannot be reduced below about 10.sup.6 cm.sup.-2. These threading dislocations propagate through reasonable thicknesses of later grown epitaxial layers, and the high defect density degrades the operation of minority carrier devices such as lasers.
In another approach, Gmitter et al. have disclosed in U.S. Pat. Nos. 4,846,931 and 4,883,561 an epitaxial lift-off procedure in which a very thin layer of a compound semiconductor crystal floating in a liquid is bonded by van der Waals force to a nearly arbitrary substrate submerged in the liquid. The van der Waals bonding occurs at room temperature and generally requires rolling pressure to remove the liquid from the interface. The strength of the van der Waals bonding is very weak compared to covalent bonding, and its reliability remains uncertain. Furthermore, epitaxial lift-off is only usable with a limited range of semiconductor composition.
In yet another approach, Liau et al. have disclosed a method of fusing together an InP wafer and a GaAs wafer in "Wafer fusion: A novel technique for optoelectronic device fabrication and monolithic integration," Applied Physics Letters, volume 56, 1990, pp. 737-739. They pressed the two wafers together with extreme pressure in the presence of phosphine (PH.sub.3) and at temperatures of at least 520.degree. C. and preferably 750.degree. C. The phosphine was probably needed to prevent the decomposition of InP and agglomeration of In globules at these high temperatures. They predicted that the temperature could be further decreased, but only if the pressure were maintained. The pressure was achieved in a jig having graphite plugs sandwiching the wafers and closely fitting inside a quartz tube. The assembly was then heated, and the differential thermal expansion produced the pressure. They did not disclose a numerical value of the fusion pressure required, but we have estimated their jig to produce at least 10 kg/cm.sup.2.
This fusion procedure requires a non-standard pressure apparatus for operation at these temperatures. The pressure is also considered to be too high to be easily achieved.